Photopolymerization is used in a variety of applications such as hardening of coated films, lithography, resin relief, and preparation of printed board, resist or photomask, black and white or color transfer development or preparation of color developing sheet. Also, in the area of dental technology, photopolymerizable compositions are used.
Photopolymerizable compositions comprise an ethylenically unsaturated compound and a photopolymerization initiator and usually they are polymerized by means of ultraviolet rays.
For example, there has been known a polymerizable composition that contains an .alpha.-ketocarbonyl compound as a photopolymerization initiator and hardens by irradiation of ultraviolet rays in the presence of amines such as N,N-dimethylaniline, which composition is used for producing a dental filler and a dental sealant, for producing a crown or a bridge, and for producing dental prosthesis (Japanese Patent Application Laid-open No. Sho 63-99858). Also, development of ultraviolet hardening ink has been made actively (Japanese Patent Application Laid-open No. Hei 2-22370).
However, such a photopolymerization as involving irradiation of ultraviolet rays has the problems that ultraviolet rays penetrate monomers insufficiently, ultraviolet rays generate ozone and cause irritation to the skin, and so on.
Accordingly, on photopolymerization initiators free of such problems, the applicant has previously filed a patent application relating to a composition that comprising a boron compound (sensitizer) that can initiate polymerization at high sensitivity with visible light and a (cationic) organic dye having an absorption in visible light region (Japanese Patent Application Laid-open No. Hei 5-59110) and a patent application relating to a composition comprising a boron compound that can initiate polymerization at high sensitivity with near-infrared light and a near-infrared light-absorbing cationic dye (Japanese Patent Application Laid-open No. Hei 5-194619).
Also, there has been developed a decolorizing agent that utilizes the phenomenon that the reaction between a dye and a boron-based compound with near-infrared light causes the color of the dye to disappear (Japanese Patent Application Laid-open No. Hei 4-362935) and it is applied to toners and inks that allow reuse of a recording material.
The above-mentioned photopolymerization initiator or decolorizing agent uses as the boron-based compound (sensitizer) a compound represented by general formula (A) ##STR2## (wherein R.sup.1A, R.sup.2A, R.sup.3A, and R.sup.4A independently represent an alkyl group, an aryl group, an alkaryl group, an allyl group, an aralkyl group, an alkenyl group, an alicyclic group or a saturated or unsaturated heterocyclic group or the like, R.sup.5A, R.sup.6A, R.sup.7A, and R.sup.8A independently represent a hydrogen atom, an alkyl group, an aryl group, an alkaryl group, an allyl group, an aralkyl group an alkenyl group, an alicyclic group, or a saturated or unsaturated heterocyclic group or the like); of these compounds, those in which at least one of R.sup.1A, R.sup.2A, R.sup.3A, and R.sup.4A represents an alkyl group and the remaining groups are each an aryl group are preferred.
As a general method of producing tetramethylammonium methyl triphenyl borate (R.sup.1A =a methyl group, R.sup.2A =R.sup.3A =R.sup.4A =a phenyl group, R.sup.5A =R.sup.6A =R.sup.7A =R.sup.8A =a methyl group), one of such preferred compounds, there has been known a method in which lithium methyl triphenyl borate obtained from triphenylborane and methyllithium is ion-exchanged with tetramethylammonium bromide [for example, Journal of the American Chemical Society, Vol. 107, page 6710 (1985)].
Also, as the method of producing triphenylborane, i.e., starting material, there has been generally known a method in which magnesium, boron trifluoride diethyl etherate, and phenyl bromide are reacted in diethyl ether [for example, Journal of Organic Chemistry, Vol. 51, page 427 (1986)].
In the case of the tetramethylammonium methyl triphenyl borate, concretely, its production is performed by reacting phenyl bromide with magnesium in diethyl ether to prepare a Grignard reagent, dripping it in a solution of boron trifluoride diethyl etherate in diethyl ether, and then stirring for several hours to obtain triphenylborane, followed by addition of methyllithium without isolating triphenylborane to convert it to lithium methyl triphenyl borate and by addition of tetramethylammonium bromide to effect ion exchange to obtain tetramethylammonium methyl triphenyl borate.
In such conventional production methods, diethyl ether is exclusively used as a solvent for a Grignard reaction or a triarylborane or trialkylborane reaction. This is because when use is made of tetrahydrofuran, in which the reaction is generally supposed to tend to occur more readily than in diethyl ether, the reaction does not stop when triaryl (or trialkyl) borane is produced but proceeds until tetraaryl (or tetraalkyl) borate is produced [for example, Journal of Organic Chemistry, Vol. 51, page 427 (1986)].
However, there arises the problem that a Grignard reaction is difficult to occur in diethyl ether depending on the type of halide to be used so that it takes a long time for the production and the final yield of the target boron-based compound is decreased.